Downhole drilling operations commonly require a downhole tool to be actuated after the tool has been deployed in the borehole. For example, underreamers are commonly tripped into the borehole in a collapsed state (i.e., with the cutting structures retracted into the underreamer tool body). At some predetermined depth, the underreamer is actuated such that the cutting structures expand radially outward from the tool body. Hydraulic actuation mechanisms are well known in oilfield services operations and are commonly employed, and even desirable, in such operations.
For example, one well-known hydraulic actuation methodology involves wireline retrieval of a plug (or “dart”) through the interior of the drill string to enable differential hydraulic pressure to actuate an underreamer. Upon completion of the reaming operation, the underreamer may be deactuated by redeploying the dart. While commercially serviceable, such wireline actuation and deactuation is both expensive and time-consuming in that it requires concurrent use of wireline or slickline assemblies.
Another commonly used hydraulic actuation methodology makes use of shear pins configured to shear at a specific differential pressure (or in a predetermine range of pressures). Ball drop mechanisms are also known in the art, in which a ball is dropped down through the drill string to a ball seat. Engagement of the ball with the seat typically causes an increase in differential pressure which in turn actuates the downhole tool. The tool may be deactuated by increasing the pressure beyond a predetermined threshold such that the ball and ball seat are released (e.g., via the breaking of shear pins). While such sheer pin and ball drop mechanisms are also commercially serviceable, they are generally one-time or one-cycle mechanisms and do not typically allow for repeated actuation and deactuation of a downhole tool.
Various other hydraulic actuation mechanisms make use of measurement while drilling (MWD) and/or other electronically controllable systems including, for example, computer controllable solenoid valves and the like. Electronic actuation advantageously enables a wide range of actuation and deactuation instructions to be executed and may further enable two-way communication with the surface (e.g., via conventional telemetry techniques). However, these actuation systems tend to be highly complex and expensive and can be severely limited by the reliability and accuracy of MWD, telemetry, and other electronically controllable systems deployed in the borehole. As a result, there are many applications ins which their use tends to be undesirable.
There remains a need in the art for a hydraulic actuation assembly that enables a downhole tool, such as an underreamer, to be actuated and deactuated substantially any number of times during a drilling operation without breaking the tool string and/or tripping the tool out of the borehole. Such an assembly is preferably purely mechanical and therefore does not require the use of electronically controllable components.